1. Field of the invention
The present invention relates to a transmission system for transmitting pictures, and more particularly is suitable for an application to a motion picture encoding method for transmitting a motion picture data into which the motion picture is compression coded.
2. Description Of the Related Arts
In a video signal transmission system for transmitting a motion picture to a remote place as in the case of, e.g., a video conference system and a video telephone system, a transmission efficiency of significant information is enhanced by interframe/intraframe-coding the video signal for utilizing a transmission path at a high efficiency.
A two-dimensional discrete cosine transformation (DCT) system may be provided as a typical coding system for coding the above-described coding data at the high efficiency.
This discrete cosine transformation system is arranged such that signal power is concentrated at specific frequency components by utilizing a two-dimensional correlation possessed by the video signals, and an information quantity is compressed by coding only coefficients distributed in concentration.
For instance, the distribution of the DCT coefficients concentrates at the low frequency components at a portion where a pattern is flat, and an auto-correlation of the video signals is high. In this case, the information quantity can be compressed by coding only the coefficients distributed in concentration at the low frequencies.
On the other hand, in a pattern having a good number of edges, the ,coefficients are generated to spread widely from the low frequency components to the high frequency components at edge discontinuous points.
In this case, an extremely large number of coefficients are required for highly accurately expressing the discontinuous points of the video signals as in the edges. This results in a drop of the coding efficiency.
At this time, if quantization characteristics of the coefficients are made rough for high-compression coding of the picture, or if the coefficients of the high frequency components are omitted, a deterioration of the video signal becomes conspicuous. For example, ringing becomes visually noticeable.
In contrast, there exists a predictive coding system as a system for coding a moving picture. This predictive coding system is capable of coding the transmission picture at a relatively high efficiency with respect to the pattern having the correlations. In the case of having less correlations, there arises a problem in which the transmission efficiency declines due to an increase in the information quantity.
To solve this, proposed is an arrangement to enhance the transmission efficiency by switching the transmission picture coding system on the block unit in PCT application No. PCT/JP92/00910.
Since only redundancy of amplitude of picture signals is compressed in the predictive coding system, however, the rate of compression is lower than that in the discrete cosine transformation system in which even a spatial frequency region is compressed. It was not to obtain higher coding efficiency, therefore, only by switching between the discrete cosine transformation system and the predictive coding system.